1. Field of the Invention
The present invention relates to a liquid crystal display device capable of halftone imaging display.
2. Description of the Related Art
FIG. 29 shows the configuration of a conventional liquid crystal display device. As shown in FIG. 29, a signal line 94 and a gate line 93 are provided and a thin-film transistor (TFT) 91 is provided in the vicinity of their crossing point. When rendered in a selected state (on-state), the thin-film transistor 91 supplies charge to an auxiliary capacitor (storage capacitor) 95 and a liquid crystal layer (liquid crystal capacitance) 92 via a pixel electrode 96. One pixel is constituted of the pixel electrode 96, a common electrode 98 confronting it, and the liquid crystal layer 92.
As is well known, to prevent deterioration of the liquid crystal layer 92, it is necessary to apply an AC voltage to the liquid crystal layer 92.
In the above type of liquid crystal display device, since it is necessary to apply an AC voltage even during a period when there are no variations in display, the potential of the pixel electrode 96 is rewritten every time the pixel is selected, that is, once per frame period.
When an AC voltage, is applied to the capacitors including the liquid crystal layer (liquid crystal capacitance) 92 and the auxiliary capacitor (storage capacitor) 95, the power consumption P is given by P=fxc3x97V2xc3x97C where f, V, and C represent the frequency, the voltage, and the total capacitance, respectively. Therefore, the power consumption increases as any of the frequency, voltage, and total capacitance increases.
In the AC driving of the liquid crystal display device, the driving frequency for each pixel is equal to the frame frequency, the driving frequency for each signal line is equal to the product of the frame frequency and the number of scanning lines, and the driving frequency of a signal line driver circuit (driver IC) 97 is equal to the product of the total number of pixels of the display screen and the frame frequency.
At present, where the liquid crystal display device is a color VGA (640xc3x973 (RGB)xc3x97480 pixels) device having a 10.4-in. diagonal size, the power consumption of the signal line driver circuit 97 is about 1 W. Therefore, in the case of an A4-size, high-resolution (corresponding to 150 dpi) liquid crystal display device, the number of pixels amounts to 1,600xc3x971,200, which is 6.25 times that of the VGA device, and hence the power consumption is as high as about 2-3 W or more.
Using a liquid crystal display device having such high power consumption as the display device of a portable information apparatus causes a problem that the usable time, which is limited by the battery performance, is shortened.
One method of solving this problem is use of a surface stabilized ferroelectric liquid crystal (SSFLC). In this case, the liquid crystal is given memorizing ability and hence the voltage supply can be stopped until a change occurs in display, which enables reduction in power consumption. However, because of the bistable nature, the liquid crystal display device basically performs binary imaging display. In this type of liquid crystal display device, it is difficult to perform halftone display and its power of expression is much lower than in a display mode capable of halftone imaging display.
Further, liquid crystals having memorizing ability are limited in display quality (contract, reflectance, etc.). For example, the display mode of the SSFLC necessarily requires polarizing plates, resulting in a small reflectance value of about 30%, which means a dark screen.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to provide a liquid crystal display device using a novel driving method which can reduce the power consumption relating to the driving and can easily provide halftone and hence perform superior halftone imaging display.
According to a first aspect of the invention, there is provided a liquid crystal display device comprising a liquid crystal layer held between a first electrode and a second electrode; means for applying an AC voltage to the first electrode or the second electrode; means for supplying a display signal; means f or selecting the display signal; means for holding the selected display signal; and an impedance element connected in series to the first electrode, an impedance of the impedance element being varied in accordance with the display signal being held.
Alternatively, there is provided a liquid crystal display device comprising a liquid crystal layer held between first electrodes and a second electrode; holding means such as a capacitor provided for each of the first electrodes, for holding a display signal; an a an impedance element connected in series to each of the first electrode and including a variable resistance element whose impedance is varied in accordance with the display signal.
As a further alternative, there is provided a liquid crystal display device comprising a first substrate on which first electrodes are arranged in matrix form; a second substrate on which a second electrode is provided; a liquid crystal layer held between the first electrodes and the second electrode; means for applying an AC voltage to each of the first electrodes or the second electrode; means for supplying a display signal; means provided for each of the first electrodes, for selecting and holding the display signal; and an impedance element connected in series to each of the first electrodes, an impedance of the impedance element being varied in accordance with the display signal being held.
For example, the first electrodes a re pixel electrodes and the second electrode is an opposed electrode (common electrode) A pixel is constituted of each of the first electrodes, the second electrode, and the liquid crystal layer held in between.
An IPS (in-plane switching) mode liquid crystal display device may be constructed by disposing the first electrodes and the second electrode on the same substrate.
The pixel electrodes may be arranged in matrix form on a substrate that is made of glass, quartz, or the like and at least the surface of which is insulating. By arranging pixels two-dimensionally in this manner, incident light on the liquid crystal layer is modulated two-dimensionally to perform display.
Where the first electrodes are pixel electrodes, covering driving elements with the pixel electrodes is preferable for the purpose of increasing the aperture ratio. Where the pixel electrodes are reflection electrodes, the invention can be applied to a reflection-type liquid crystal display device. In this case, since the selecting means and the holding means can be formed under the reflection electrode, the degree of design freedom increases even if the driving element is increased in size.
The display signal is a signal for controlling the states of the pixels, that is, the states of the liquid crystal layer held between the first electrodes and the second electrode.
For example, the means for supplying such a display signal is signal supply lines. A plurality of display signal supplying means may be provided rather than a single one.
The means for selecting a display signal is means for performing, on apixel-by-pixelbasis, selection/sampling onthe display signal that is supplied in the above manner. For example, a nonlinear switching element such as a thin-film transistor whose source or drain is connected to a signal line may be used as the display signal selecting means. By controlling the gate electrode of the thin-film transistor by a scanning signal, a display signal can be captured independently for each of arbitrary pixels.
Where the display signal is supplied to the pixels as digital data, a sampling circuit may be constructed by combining, for example, logic gates, data latches, a shift register, etc.
For example, the impedance element may be a parallel connection of a plurality of series connections of a variable resistance element and a capacitance element.
The plurality of capacitance values may be so set that their combinations provide gradation voltages that vary smoothly.
It is preferable that the capacitance of the impedance element be so set as to be smaller than a capacitance formed by each of the first electrode, the second electrode, and the liquid crystal layer held in between, that is, a liquid crystal capacitance.
A capacitive load as viewed from the display signal supply side when a display signal is held that includes the display signal holding means for each pixel and the impedance element may be set smaller than the liquid crystal capacitance.
The variable resistance elements may be given different impedance values in accordance with a display signal being held by the display signal holding means that is provided for each pixel.
For example, the variable resistance element may be a three-terminal element such as a thin-film transistor. Not only is the variable resistance element used as a switching element for an on/off control but also its middle resistance value may be used.
The plurality of capacitance values may be so set that combinations of selected ones of the capacitance values correspond to smoothly varied gradation levels.
According to the first aspect of the invention, since the impedance element that is connected in series to the liquid crystal capacitance is a parallel connection of a plurality of series connections of a variable resistance element and a capacitance element, a voltage that is applied to the liquid crystal layer constituting each pixel can be controlled digitally by controlling the states of the variable resistance elements. Therefore, it becomes possible to display halftone in a stable manner.
By making the impedance values of the respective variable resistance elements different from each other in accordance with a display signal, the liquid crystal application voltage can be varied gently in accordance with a display signal stored in each pixel. This enables a correct halftone control even in the case of analog-like halftone display.
Further, according to the first aspect of the invention, a display signal is written from a signal line to the holding means such as an auxiliary capacitor rather than charge is directly written from the signal line to the liquid crystal layer as in the conventional case. Therefore, if the auxiliary capacitance is set smaller than the liquid crystal capacitance, the load capacitance of the signal line of each pixel is reduced but also a display signal can be written quickly at the time of pixel selection, which shorten the pixel selection time. Therefore, the first aspect of the invention makes it possible to increase the screen size and the resolution of a liquid crystal display device.
According to a second aspect of the invention, there is provided a liquid crystal display device comprising a liquid crystal layer held between a first electrode and a second electrode; means for supplying a display signal; means for selecting the display signal; a variable capacitance element connected in series to the first electrode, a capacitance of the variable capacitance element being varied in accordance with the selected display signal; first applying means for applying a first AC voltage; second applying means for applying a second AC voltage that is different in amplitude from the first AC voltage; and switching means for applying the first AC voltage or the second AC voltage to the first electrode or the second electrode via the variable capacitance element in accordance with the selected display signal.
The variable capacitance element may comprise a parallel connection of a plurality of series connections of a switching element and a capacitor, and the capacitance of the variable capacitance element may be varied by turning on or off the switching elements in accordance with the selected display signal.
The switching means may be a decoder that on/off-controls the switching elements in accordance with the display signal.
A first gradation range of the liquid crystal layer that is displayed through application of the first AC voltage may be made continuous with a second gradation range of the liquid crystal layer that is displayed through application of the second AC voltage.
According to a third aspect of the invention, there is provided a liquid crystal display device comprising a first liquid crystal layer held between a first electrode and a second electrode; a second liquid crystal layer held between the second electrode and a third electrode and laid on the first liquid crystal layer; a third liquid crystal layer held between the third electrode and an opposed electrode and laid on the second liquid crystal layer; first applying means for applying a first AC voltage; second applying means for applying a second AC voltage; third applying means for applying a third AC voltage; a first variable capacitance element interposed between the first applying means and the first electrode, a capacitance of the first variable capacitance element being varied in accordance with a first display signal; a second variable capacitance element interposed between the second applying means and the second electrode, a capacitance of the second variable capacitance element being varied in accordance with a second display signal; and a third variable capacitance element interposed between the third applying means and the third electrode, a capacitance of the third variable capacitance element being varied in accordance with a third display signal, wherein voltages produced by dividing the first, second, and third AC voltages by the first, second, and third variable capacitance elements and the first, second, and third liquid crystal layers are applied to the first, second, and third electrodes, respectively.
In the above liquid crystal display device, the first, second, and third AC voltages may have the same frequency and phases of AC voltages that are applied to a plurality of pixel electrodes that constitute a unit pixel may be equalized for the unit pixel.
Each of the first, second, and third variable capacitance elements may comprise a parallel connection of a plurality of series connections of a switching element and a capacitor, and the capacitances of the first, second, and third variable capacitance elements may be varied by turning on or off the switching elements in accordance with the first, second, and third display signals, respectively.
A first gradation range of the first, second, or third liquid crystal layer that is displayed through application of the first AC voltage may be made continuous with a second gradation range of the first, second, or third liquid crystal layer that is displayed through application of the second AC voltage.
The first and second liquid crystal layers may be made of guest-host liquid crystals containing dyes of different colors and laid one on anther to constitute a unit pixel.
Alternatively, there is provided a liquid crystal display device in which a plurality of liquid crystal layers are laid one on another and intermediate electrodes are provided, comprising a variable capacitance forming sections each including a plurality of capacitors and switches for selection of the capacitances; liquid crystals corrected to the respective variable capacitance forming sections (and driven by respective pixel electrodes); and a pixel circuit for applying AC voltages and controlling the AC voltages to be applied to the respective liquid crystals through capacitive voltage division, wherein the AC voltages have the same frequency and phases of AC voltages that are applied to a plurality of pixel electrodes that constitute a unit pixel are equalized for the unit pixel.
According to the second and third aspects of the invention, in a liquid crystal display device having a plurality of pixels, a variable capacitance element provided in each pixel and constituted of a plurality of capacitors and switches for selection of those capacitors, a liquid crystal connected to the variable capacitance element (and driven by a pixel electrode) , and a pixel circuit for applying an AC voltage and controlling an AC voltage to be applied to the pixel through capacitive voltage division, a circuit is further provided that selects among a plurality of AC voltages having different amplitudes and applies a selected AC voltage to the variable capacitance forming section.
For example, the variable capacitance element may be a parallel connection of a plurality of series connections of a capacitance element and a switch. A capacitance corresponding to a display signal can be formed by controlling, in accordance with the display signal, the number of switches to be turned on.
Since an AC voltage applied by the applying means is divided by the liquid crystal capacitance of the pixel and the capacitance of the variable capacitance element that varies in accordance with a display signal, an AC signal that is controlled in accordance with the display signal is applied to the liquid crystal layer.
The number of combinations of capacitors that constitute the variable capacitance element may be set larger than the desired number of display gradation levels.
An auxiliary capacitor may be so provided as to be connected in parallel to the liquid crystal capacitance in an AC-like manner.
The first AC voltage and the second AC voltage may be different amplitudes. AC voltages having different phases depending on the pixel may be applied. However, in a case where a plurality of liquid crystal layers are laid one on another and a unit pixel is constituted of a plurality of laminated pixels as in the case of a three-layer GH liquid crystal display device, for example, it is necessary that the first and secondAC voltages have the same phase difference within the unit pixel. For example, both of the frequencies and the phases of the first and second AC voltages may be equalized.
The switching means is to selectively apply the first or second AC voltage to the first or second electrode in accordance with a display signal, and may be made of switching elements such as thin-film transistors.
According to the liquid crystal display device, by employing the above configurations, the relationship between the gradation level of a displayed image and the voltage that is actually applied to the liquid crystal layer constituting each pixel can be corrected in consideration of the electro-optical response characteristic (transmittance/reflectance or the like) of the liquid crystal layer and the luminous efficiency, whereby superior halftone imaging display can be realized.